Polarization-mode-dispersion (PMD) is a common phenomenon that occurs when light waves travel in optical media such as optical fiber and optical amplifiers. PMD occurs in an optical fiber as a result of small birefringence induced by deviations of the fiber's core from a perfectly cylindrical shape, asymmetric stresses or strains, and/or random external forces acting upon the fiber. PMD causes the two orthogonal polarization components of an optical signal corresponding to two principle states of polarization (PSP) of a transmission link to travel at different speeds and arrive at a receiver with a differential group delay (DGD). As a result, the waveform of optical signals may be significantly distorted, resulting in more frequent errors at the receiver.
PMD is wavelength-dependent in that the amount or level of PMD imparted by an optical component (e.g., optical fiber) at a given time will generally vary for different wavelength-division-multiplexing (WDM) channels corresponding to different signal wavelengths or frequencies.
Polarization-dependent loss (PDL) is another common phenomenon in optical fiber transmission. Optical components such optical add/drop modules (OADM's) tend to have PDL, which attenuate optical signals depending on the relative polarization state with respect to the PSP's of the PDL component.
Polarization-dependent gain (PDG) is also a common phenomenon in optical fiber transmission. Optical components such as Erbium-doped fiber amplifiers (EDFAs), tend to have PDG, which amplify optical signals depending on their relative polarization state with respect to the PSPs of the PDG component. PDL and PDG cause signals to have different amplitudes at the receiver, which makes the optimal decision threshold different for different bits (depending on their polarization), and thus degrades the receiver performance when the receiver decision threshold can only be fixed to a certain level for all the bits. PDL may also cause varying optical signal-to-noise-ratio (OSNR) for bits with different polarization, and further degrade the system performance. PDL or PDG induced OSNR degradation cannot be compensated for since the process of adding random amplified spontaneous emission (ASE) noise cannot be undone.
It is known that PMD, PDL, and PDG are significant penalty sources in high-speed (e.g., 10 Gb/s and 40 Gb/s) transmissions. PMD compensation (PMDC) is normally desirable to increase system tolerance to PMD. However, due to the stochastic nature of PMD and its wavelength dependence, PMDC is normally required to be implemented for each wavelength channel individually, and is thus generally not cost-effective. Various prior art methods have been proposed to achieve PMDC simultaneously for multiple WDM channels. Channel switching is one technique that has been proposed to mitigate the overall PMD penalty in a WDM system. However, such systems sacrifice system capacity due to the use of extra channels for PMD protection. Multi-channel PMDC before wavelength de-multiplexing has also been proposed to mitigate the PMD degradation in the WDM channel having the most severe PMD. However, such a mitigation scheme may cause degradation of other channels.
Another scheme for a multi-channel shared PMDC has been proposed in which the most degraded channel is switched, by optical or electrical means, to a path connected to the shared PMDC; however, the speed of PMDC is limited (by the speed of the optical or electrical switching). In current PMDC schemes, PMD induced system outages, during which the PMD penalty exceeds its pre-allocated system margin and system failure occurs, are present, though reduced.
Forward-error-correction (FEC) is an effective technique for increasing system margin cost-effectively. It has been determined, however, that FEC cannot extend the tolerable PMD for a fixed PMD penalty at a given average bit-error-rate (BER), even though the additional margin provided by FEC can be used to increase the PMD tolerance. It has been suggested that sufficient interleaving in FEC may increase PMD tolerance. However, there is no known practical method to provide the deep interleaving needed to avoid a PMD outage which may last minutes or longer in practical systems.